id go for the second hand box mate, then get the garage that is fitting it to look over it and also your getting a warrenty with it, cant believe a garage dont put warrenty on there work, thats complete Sh*t if something is going to happen with this second hand box it will be just after you have fitted it then will be still in warrenty ask them if the box does break who will pay for labour to take it off and a new one on again!!!
Not defending the garage here but they do offer a warrenty on there work, its just that mine is not standard spec so there not prepared to offer one. I can understand where there coming from.